Near-surface temperature and specific humidity from PRISM & 4 reanalyses are used to quantify historical trends in frequency (freq), intensity (int.) & spatial extent (area) of extreme T-only & T & TE extreme heat days (EHD). The southwest and southern Great Plains exhibit strong positive trends in freq./int./area T-only EHD. All other regions exhibit strongest positive trends in freq/int./area T&TE EHD.
Impacts from extreme heat (T) depend on specific humidity (Q). Dry heat (high T only) leads to agricultural losses. Wet heat (high T & high TE) leads to excess human mortality & morbidity. Thus, we need to use composite variables (e.g. equivalent temperature that combines T & Q) to characterize heat waves. TE is also an integrative metric of static energy & at the energy-water-land nexus, it depends on sensible v. latent heat fluxes).
Our research illustrates long-term tendencies in EHD and shows events associated with human health impacts are increasing in frequency, intensity, and area affected.